Conventionally, a tuning fork crystal unit, which can be easily made small, has been known as a type of piezoelectric device. This type of crystal unit is disclosed in, for example, JP 2002-76806A. The crystal unit comprises a tuning fork crystal resonator that is produced by etching a crystal wafer into a shape of a tuning fork and forming a predetermined electrode on a surface thereof by utilizing a photolithography technique. A groove is formed at a middle portion of each of top and bottom surfaces of each leg portion (leg portion major surface) of the tuning fork crystal resonator. When the groove portion is thus formed on the top and bottom surfaces of the leg portion, even a small resonator can suppress a loss of vibration in the leg portion, thereby making it possible to suppress the crystal impedance (CI) value to a small level. Thus, the groove portion is effective. This type of tuning fork crystal unit is particularly suitably mounted on precision apparatuses, such as a clock and the like.
A shape of the electrode formed on the surface of this type of tuning fork crystal resonator will be hereinafter described.
FIG. 9 shows a tuning fork crystal resonator a that is included in a general tuning fork crystal unit. The tuning fork crystal resonator a comprises two leg portions b, c. First drive electrodes d1, d2 and second drive electrodes e1, e2 are formed in each of the leg portions b, c. In FIG. 9, portions in which the drive electrodes d1, d2, e1, e2 are formed are hatched.
Also in the tuning fork crystal resonator a, rectangular groove portions b2, c2 are formed on leg portion major surfaces b1, c1, which are top and bottom surfaces of the leg portions b, c, respectively.
The first drive electrode is composed of an in-groove electrode d1 that is formed in a groove portion b2 formed on the top and bottom surfaces (leg portion major surface) b1 of the leg portion b, and a side surface electrode d2 that is formed on a side surface c3 of the leg portion c. The in-groove electrode d1 and the side surface electrode d2 are connected via a connecting electrode f.
Similarly, the second drive electrode is composed of an in-groove electrode e1 that is formed in a groove portion c2 formed on the top and bottom surfaces (leg portion major surface) c1 of the leg portion c, and a side surface electrode e2 that is formed on a side surface b3 of the leg portion b. The in-groove electrode e1 and the side surface electrode e2 are connected via a connecting electrode g.
These electrodes are a thin film composed of, for example, a chromium (Cr) underlying electrode layer and a gold (Au) top electrode layer. The film is formed on the entire surface using a technique, such as vacuum deposition or the like, and is then formed into a desired shape by metal etching using a photolithography technique.
Note that the formation of the electrode using vacuum deposition has the following benefit. The thicknesses of the underlying electrode layer (chromium) and the top electrode (gold) can be easily controlled, resulting in desired electrical characteristics. Also, by controlling a deposition apparatus in an appropriate manner, high film quality can be obtained.
In the above-described tuning fork crystal resonator, the in-groove electrode is formed from a bottom surface of the groove portion to a vertical wall (a surface perpendicular to the leg portion major surface) and the leg portion major surface. Therefore, a connection between the in-groove electrode and the connecting electrode is unstable in the vicinity of an edge (a border between the groove portion and the leg portion major surface), likely leading to a break. Particularly, when the electrode is formed using vacuum deposition, the bottom surface and the vertical wall (a surface perpendicular to the leg portion major surface) of the groove portion closer to a base portion of the resonator are likely to be blocked from the view of a deposition source, so that the connection between the in-groove electrode and the connecting electrode is unstable and a break is likely to occur.
In addition, when the photolithography technique is used to form the in-groove electrode in the groove portion, the surface of a resist liquid is bulged upward due to its surface tension in the vicinity of an edge of the groove portion (a border between the groove portion and the leg portion major surface), resulting in a reduction in etching precision of the in-groove electrode. In this case, since an outer edge of the in-groove electrode is not appropriately shaped, a break is likely to occur between the in-groove electrode and the connecting electrode, or the in-groove electrode is made contact with the connecting electrode or the side surface electrode at a crotch portion of the tuning fork crystal resonator, resulting in occurrence of a short circuit.
These problems are significant in a micro-crystal unit (e.g., the width of the leg portion is about 120 μm) that has been recently much developed.
As described above, in the tuning fork crystal resonator in which the leg portion has a groove portion and an in-groove electrode is provided in the groove portion, an electrical failure, such as a break in the electrode, a short circuit between the electrodes or the like, is likely to occur due to the presence of the groove portion and the in-groove electrode. Therefore, a further improvement in structure is required for the above-described type of tuning fork crystal resonator.
The present invention is provided to solve the problems. An object of the present invention is to provide a tuning fork crystal resonator and a tuning fork unit in which a groove portion is provided in a leg portion thereof and an drive electrode (in-groove electrode) is formed in the groove portion and that can avoid an electrical failure that is otherwise caused by the electrode formed on a surface of the resonator.